The present invention relates to an improvement in a slit exposure optical system which is to be used in a copying machine or the like.
As is well known, in a copying machine, a slit exposure optical system having two movable tables is used in order to constantly maintain an optical path length of an original image incident upon a projection lens. Specifically, a slit exposure optical system of this kind is equipped with a primary movable table having a slit for scanning the surface of an original and a primary mirror, and a secondary movable table carrying a secondary mirror which receives an incident ray of light from the primary mirror, and a motor-driven wire sheave means is suspended between the respective one end of said movable tables; the secondary movable table is driven by said wire sheave means at a half of the speed of the primary movable table.
In such a slit exposure optical system as described above, it is necessary that the mirrors on the primary and the secondary movable tables should always be at right angles to the moving direction of the movable tables. However, if the primary and the secondary movable tables are driven only at one ends thereof, the other ends thereof are delayed in motion in the backward direction of the movement thereof, and an aberration is thus caused between both ends of the optical axes of the mirror. The above phenomenon becomes a problem particularly in large-sized copying machines in which the movable table is becoming larger in size and heavier in weight, or in high-speed copying machines in which there is a sudden acceleration or deceleration of the movable table. To solve this problem, in a conventional type of a large-sized copying machines, two pairs of wire sheave means are arranged respectively between the both ends of the movable tables, and a driving force is applied to the both wire sheave means, and the both movable tables are thus driven. However, the arrangements of two pairs of wire sheave means for driving use as mentioned above complicates the driving system and increases the manufacturing costs thereof.